Evolution of plant defenses in nonindigenous environments

How generalist insect herbivores respond to alien plants? The case of Aphis fabae-Myzus persicae-Rhododendron ponticum

BACKGROUND

The enemy release hypothesis (ERH) predicts that alien plant species are unsuitable hosts for native phytophagous insects. However, the biotic resistance hypothesis (BRH) predicts that generalist herbivores may prefer an alien plant over their common host plant. In this study, we have tested these two hypotheses by comparing the potential colonization of the invasive Pontic rhododendron (Rhododendron ponticum L.) versus the common rearing host plants by two generalist aphid species (Aphis fabae and Myzus persicae). We assessed (i) the probing behavior using the electrical penetration graph (EPG) technique and (ii) survival and fecundity in Petri dishes.

RESULTS

The results showed the inability of A. fabae and Myzus persicae to immediately colonize R. ponticum. Despite their ability to feed on this invasive plant, the two aphid species hardly survived and poorly reproduced.

CONCLUSION

Our results are consistent with the ERH, since R. ponticum appeared as an unsuitable host for native phytophagous insects. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Complementary roles of two classes of defense chemicals in white spruce against spruce budworm

MAIN CONCLUSION

Monoterpenes and phenolics play distinct roles in defending white spruce trees from insect defoliators. Monoterpenes contribute to the toxicity of the foliage, deterring herbivory, whereas phenolics impede budworm growth. This study demonstrates the complex interplay between monoterpenes and phenolics and their collective influence on the defense strategy of white spruce trees against a common insect defoliator. Long-lived coniferous trees display considerable variations in their defensive chemistry. The impact of these defense phenotype variations on insect herbivores of the same species remains to be thoroughly studied, mainly due to challenges in replicating the comprehensive defense profiles of trees under controlled conditions. This study methodically examined the defensive properties of foliar monoterpenes and phenolics across 80 distinct white spruce families. These families were subsequently grouped into two chemotypes based on their foliar monoterpene concentrations. To understand the separate and combined effects of these classes on tree defenses to the eastern spruce budworm, we conducted feeding experiments using actual defense profiles from representative families. Specifically, we assessed budworm response when exposed to substrates amended with phenolics alone or monoterpenes. Our findings indicate that the ratios and amounts of monoterpenes and phenolics present in the white spruce foliage influence the survival of spruce budworms. Phenotypes associated with complete larval mortality exhibited elevated ratios (ranging from 0.4 to 0.6) and concentrations (ranging from 1143 to 1796 ng mg-1) of monoterpenes. Conversely, families characterized by higher phenolic ratios (ranging from 0.62 to 0.77) and lower monoterpene concentrations (ranging from 419 to 985 ng mg-1) were less lethal to the spruce budworm. Both classes of defense compounds contribute significantly to the overall defensive capabilities of white spruce trees. Monoterpenes appear critical in determining the general toxicity of foliage, while phenolics play a role in slowing budworm development, thereby underscoring their collective importance in white spruce defenses.

Trade-offs in non-native plant herbivore defences enhance performance

Non-native plants are typically released from specialist enemies but continue to be attacked by generalists, albeit at lower intensities. This reduced herbivory may lead to less investment in constitutive defences and greater investment in induced defences, potentially reducing defence costs. We compared herbivory on 27 non-native and 59 native species in the field and conducted bioassays and chemical analyses on 12 pairs of non-native and native congeners. Non-natives suffered less damage and had weaker constitutive defences, but stronger induced defences than natives. For non-natives, the strength of constitutive defences was correlated with the intensity of herbivory experienced, whereas induced defences showed the reverse. Investment in induced defences correlated positively with growth, suggesting a novel mechanism for the evolution of increased competitive ability. To our knowledge, these are the first linkages reported among trade-offs in plant defences related to the intensity of herbivory, allocation to constitutive versus induced defences, and growth.

Landscape configuration modulates the presence of leaf-cutting ants in eucalypt plantations

Pest responses to landscape complexity show variable patterns globally, primarily related to species traits and specific managed habitats. Leaf-cutting ants (LCAs) are native insects and important pests of plantation forests in South America. We evaluated the responses of LCA nests in young Eucalyptus plantations to different spatial contexts: land uses, interfaces (adjacent land use pairs), agroecosystems, and landscapes. We selected 30 sites in the littoral region of Argentina representing three types of land uses neighboring Eucalyptus plantations: adult eucalypt plantations, citrus plantations, and semi-natural habitats. At each site, we quantified and identified LCA nests and characterized landscape composition and configuration in circles of 250 m radius. LCA nest abundance and presence were similar across different land uses, interfaces, and agroecosystems. Nest presence decreased in landscapes with increasing mean perimeter/area ratio and citrus coverage, whereas LCA abundance showed a similar trend. This indicates that heterogeneous landscapes and those with greater citrus plantation coverage were less likely to have LCA nests. Our findings suggest that landscape configuration was the main predictor of the LCA presence. Understanding the dynamics of LCAs populations and their complex associations with landscape components will contribute to developing successful environmental pest management strategies for plantation forests.

Plant-soil feedback from eastern redcedar (Juniperus virginiana) inhibits the growth of grasses in encroaching range

The encroachment of woody plants into grasslands is an ongoing global problem that is largely attributed to anthropogenic factors such as climate change and land management practices. Determining the mechanisms that drive successful encroachment is a critical step towards planning restoration and long-term management strategies. Feedbacks between soil and aboveground communities can have a large influence on the fitness of plants and must be considered as potentially important drivers for woody encroachment. We conducted a plant-soil feedback experiment in a greenhouse between eastern redcedar Juniperus virginiana and four common North American prairie grass species. We assessed how soils that had been occupied by redcedar, a pervasive woody encroacher in the Great Plains of North America, affected the growth of Andropogon gerardi, Schizachyrium scoparium, Bromus inermis, and Pascopyrum smithii over time. We evaluated the effect of redcedar on grass performance by comparing the height and biomass of individuals that were grown in live or sterilized conspecific or redcedar soil. We found redcedar created a negative plant-soil feedback that limited the growth of the cool season grasses B. inermis and P. smithii, reducing their overall biomass by >60%. These effects were found in both live and sterilized redcedar soils. In live soils, some growth suppression can be attributed to the negative effects of soil microbes. The limitation of grass growth in sterile soils indicates redcedar may exude an allelochemical into the soil that limits grass growth. Our results demonstrate that plant-soil feedback created by redcedar inhibits the growth of certain grass species. By creating a plant-plant interaction that negatively affects competitors, redcedars increase the probability of seedling survival until they can grow to overtop their neighbors. These results indicate plant-soil feedback is a mechanism of native woody plant encroachment which could be important in many systems yet is understudied.

Can the enemy release hypothesis explain the success of Rumex (Polygonaceae) species in an introduced range?

The enemy release hypothesis states that introduced plants have a competitive advantage due to their release from co-evolved natural enemies (i.e., herbivores and pathogens), which allows them to spread rapidly in new environments. This hypothesis has received mixed support to date, but previous studies have rarely examined the herbivore community, plant damage, and performance simultaneously and largely ignored below-ground herbivores. We tested for enemy release by conducting large scale field surveys of insect diversity and abundance in both the native (United Kingdom) and introduced (New Zealand) ranges of three dock (Rumex, Polygonaceae) species: R. conglomeratus Murray (clustered dock), R. crispus L. (curly dock) and R. obtusifolius L. (broad-leaved dock). We captured both above- and below-ground insect herbivores, measured herbivore damage, and plant biomass as an indicator for performance. In the introduced range, Rumex plants had a lower diversity of insect herbivores, all insect specialists present in the native range were absent and plants had lower levels of herbivore damage on both roots and leaves. Despite this, only R. crispus had greater fresh weight in the introduced range compared to the native range. This suggests that enemy release, particularly from below-ground herbivores, could be a driver for the success of R. crispus plants in New Zealand, but not for R. conglomeratus and R. obtusifolius.

Phylogenetic patterns suggest frequent multiple origins of secondary metabolites across the seed-plant 'tree of life'

To evaluate the phylogenetic patterns of the distribution and evolution of plant secondary metabolites (PSMs), we selected 8 classes of PSMs and mapped them onto an updated phylogenetic tree including 437 families of seed plants. A significant phylogenetic signal was detected in 17 of the 18 tested seed-plant clades for at least 1 of the 8 PSM classes using the D statistic. The phylogenetic signal, nevertheless, indicated weak clustering of PSMs compared to a random distribution across all seed plants. The observed signal suggests strong diversifying selection during seed-plant evolution and/or relatively weak evolutionary constraints on the evolution of PSMs. In the survey of the current phylogenetic distributions of PSMs, we found that multiple origins of PSM biosynthesis due to external selective forces for diverse genetic pathways may have played important roles. In contrast, a single origin of PSMs seems rather uncommon. The distribution patterns for PSMs observed in this study may also be useful in the search for natural compounds for medicinal purposes.

Genetically Based Trait Differentiation but Lack of Trade-offs between Stress Tolerance and Performance in Introduced Canada Thistle

Trade-offs between performance and tolerance of abiotic and biotic stress have been proposed to explain both the success of invasive species and frequently observed size differences between native and introduced populations. Canada thistle seeds collected from across the introduced North American and the native European range were grown in benign and stressful conditions (nutrient stress, shading, simulated herbivory, drought, and mowing), to evaluate whether native and introduced individuals differ in performance or stress tolerance. An additional experiment assessed the strength of maternal effects by comparing plants derived from field-collected seeds with those derived from clones grown in the glasshouse. Introduced populations tended to be larger in size, but no trade-off of stress tolerance with performance was detected; introduced populations had either superior performance or equivalent trait values and survivorship in the treatment common gardens. We also detected evidence of parallel latitudinal clines of some traits in both the native and introduced ranges and associations with climate variables in some treatments, consistent with recent climate adaptation within the introduced range. Our results are consistent with rapid adaptation of introduced populations, but, contrary to predictions, the evolution of invasive traits did not come at the cost of reduced stress tolerance.

Are there phylogenetic differences in salivary tannin-binding proteins between browsers and grazers, and ruminants and hindgut fermenters?

While feeding, mammalian browsers (primarily eat woody plants) encounter secondary metabolites such as tannins. Browsers may bind these tannins using salivary proteins, whereas mammalian grazers (primarily eat grasses that generally lack tannins) likely would not. Ruminant browsers rechew their food (ruminate) to increase the effectiveness of digestion, which may make them more effective at binding tannins than nonruminants. Few studies have included a sufficient number of species to consider possible scaling with body mass or phylogenetic effects on salivary proteins. Controlling for phylogeny, we ran inhibition radial diffusion assays of the saliva of 28 species of African herbivores that varied in size, feeding strategy, and digestive system. We could not detect the presence of salivary proline-rich proteins that bind tannins in any of these species. However, using the inhibition radial diffusion assay, we found considerable abilities to cope with tannins in all species, albeit to varying degrees. We found no differences between browsers and grazers in the effectiveness of their salivary proteins to bind to and precipitate tannins, nor between ruminants and nonruminants, or scaling with body mass. Three species bound all tannins, but their feeding niches included one browser (gray duiker), one mixed feeder (bush pig), and one grazer (red hartebeest). Five closely related species of small ruminant browsers were very effective in binding tannins. Megaherbivores, considered generalists on account of their large body size, were capable of binding tannins. However, the grazing white rhinoceros was almost as effective at binding tannins as the megaherbivore browsers. We conclude, contrary to earlier predictions, that there were no differences in the relative salivary tannin-binding capability that was related to common ancestry (phylogeny) or to differences in body size.

Photochemical efficiency is negatively correlated with the Δ9- tetrahydrocannabinol content in Cannabis sativa L

Cannabis sativa L is an important plant, which is a source of durable fibers, nutritious seeds, and medicinally important phytocannabinoids including Δ⁹-tetrahydrocannabinol (THC) and cannabidiol (CBD). Light has shown to be a key modulator of biomass and cannabinoid yield suggesting responsive photochemical machinery. The present study was envisaged to understand the effect of the increasing levels of metabolic THC on the photochemical efficiency in Cannabis. The chlorophyll a fluorescence kinetics, photosynthetic pigments and immuno-detection of the photosynthetic machinery was analyzed on seven accessions from different environments, in conjunction with the cannabinoid content. All the accessions were clearly divided into three groups based on their relative content of CBD and THC. Group I with (CBD/THC > 1) had a clear advantage in terms of the damage to the D1, RbCL and Lhc1 protein holo-complex. Performance indicators of photochemistry based on the OJIP kinetics suggested a stoichiometrically negative correlation with the THC content. Zeaxanthin-dependent quenching is primarily responsible for lower NPQ in Group III with high THC content (THC > 6%). The THC treatment on Arabidopsis thaliana also suggested dose-dependent decrease in the photochemical efficiency suggesting the exclusivity of THC in causing the response. This resulted in the damage of photosynthetic machinery and the generation of free radicals, thereby compromising the yield. The study also opens a new screening method for Cannabis, based on cannabinoid content.

Rapid evolution of Medicago polymorpha during invasion shifts interactions with the soybean looper

The Enemy Release Hypothesis posits that invasion of novel habitats can be facilitated by the absence of coevolved herbivores. However, a new environment and interactions with unfamiliar herbivores may impose selection on invading plants for traits that reduce their attractiveness to herbivores or for enhanced defenses compared to native host plants, leading to a pattern similar to enemy release but driven by evolutionary change rather than ecological differences. The Shifting Defense Hypothesis posits that plants in novel habitats will shift from specialized defense mechanisms to defense mechanisms effective against generalist herbivores in the new range. We tested these ideas by comparing herbivore preference and performance of native (Eurasia)- and invasive (New World)-range Medicago polymorpha, using a generalist herbivore, the soybean looper, that co-occurs with M. polymorpha in its New World invaded range. We found that soybean loopers varied in preference and performance depending on host genotype and that overall the herbivore preferred to consume plant genotypes from naïve populations from Eurasia. This potentially suggests that range expansion of M. polymorpha into the New World has led to rapid evolution of a variety of traits that have helped multiple populations become established, including those that may allow invasive populations to resist herbivory. Thus, enemy release in a novel range can occur through rapid evolution by the plant during invasion, as predicted by the Shifting Defense Hypothesis, rather than via historical divergence.

Greater phylogenetic distance from native oaks predicts escape from insect leaf herbivores by non-native oak saplings

PREMISE

Non-native plant species have been hypothesized to experience lower herbivory in novel environments as a function of their phylogenetic distance from native plant species. Although recent work has found support for this prediction, the plant traits responsible for such patterns have been largely overlooked.

METHODS

In a common garden experiment in northwestern Spain, we tested whether oak species (Quercus spp.) not native to this region that are phylogenetically more distantly related to native species exhibit less insect leaf herbivory. In addition, we also investigated plant traits potentially correlated with any such effect of phylogenetic distance.

RESULTS

As expected, phylogenetic distance from native species negatively predicted insect leaf herbivory on non-native oaks. In addition, we found that the leaf traits, namely phosphorus and condensed tannins, were significantly associated with herbivory, suggesting that they are associated with the effect of phylogenetic distance on leaf herbivory on non-native oak species.

CONCLUSIONS

This study contributes to a better understanding of how evolutionary relationships (relatedness) between native and non-native plant species determine the latter's success in novel environments via locally shared enemies, and encourages more work investigating the plant traits that mediate the effects of phylogenetic distance on enemy escape.

Rapid growth and defence evolution following multiple introductions

Rapid adaptation can aid invasive populations in their competitive success. Resource allocation trade-off hypotheses predict higher resource availability or the lack of natural enemies in introduced ranges allow for increased growth and reproduction, thus contributing to invasive success. Evidence for such hypotheses is however equivocal and tests among multiple ranges over productivity gradients are required to provide a better understanding of the general applicability of these theories.Using common gardens, we investigated the adaptive divergence of various constitutive and inducible defence-related traits between the native North American and introduced European and Australian ranges, while controlling for divergence due to latitudinal trait clines, individual resource budgets, and population differentiation, using >11,000 SNPs.Rapid, repeated clinal adaptation in defence-related traits was apparent despite distinct demographic histories. We also identified divergence among ranges in some defence-related traits, although differences in energy budgets among ranges may explain some, but not all, defence-related trait divergence. We do not identify a general reduction in defence in concert with an increase in growth among the multiple introduced ranges as predicted trade-off hypotheses. Synthesis: The rapid spread of invasive species is affected by a multitude of factors, likely including adaptation to climate and escape from natural enemies. Unravelling the mechanisms underlying invasives' success enhances understanding of eco-evolutionary theory and is essential to inform management strategies in the face of ongoing climate change.

OPEN RESEARCH BADGES

This article has been awarded Open Materials, Open Data, Preregistered Research Designs Badges. All materials and data are publicly accessible via the Open Science Framework at https://doi.org/10.6084/m9.figshare.8028875.v1, https://github.com/lotteanna/defence_adaptation,https://doi.org/10.1101/435271.

Testing the Enemy Release Hypothesis on tall-statured grasses in South Africa, using Arundo donax, Phragmites australis, and Phragmites mauritianus as models

The Enemy Release Hypothesis (ERH) predicts that introduced plant species can escape herbivory and therefore have a competitive advantage over native plants, which are exposed to both generalist and specialist natural enemies. In this study, the ERH was explored using the invasive alien species, Arundo donax and two native tall-statured grasses, the cosmopolitan Phragmites australis and African endemic Phragmites mauritianus in South Africa. It was predicted that A. donax would have reduced species richness of herbivores compared with the native Phragmites spp., that it would be devoid of specialist herbivores and would thus be experiencing enemy escape in the adventive range. The herbivore assemblages were determined from both field surveys and a literature review. The assumptions of the ERH were for the most part not met; 13 herbivores were found on A. donax compared with 17 on P. australis and 20 on P. mauritianus. Arundo donax had two specialist herbivores from its native range, and shared native herbivores with Phragmites spp. Although A. donax had reduced species richness and diversity compared with that found in the native distribution, it has partially re-acquired a herbivore assemblage which is similar to that found on analogous native species. This suggests that enemy release may not fully explain the invasive success of A. donax in South Africa.

Diversity analysis of the rhizospheric and endophytic bacterial communities of Senecio vulgaris L. (Asteraceae) in an invasive range

Increasing evidence has confirmed the importance of plant-associated bacteria for plant growth and productivity, and thus it is hypothesized that interactions between bacteria and alien plants might play an important role in plant invasions. However, the diversity of the bacterial communities associated with invasive plants is poorly understood. We therefore investigated the diversity of rhizospheric and endophytic bacteria associated with the invasive annual plant Senecio vulgaris L. (Asteraceae) based on 16S rRNA gene data obtained from 57 samples of four Senecio vulgaris populations in a subtropical mountainous area in central China. Significant differences in diversity were observed between plant compartments. Specifically, the rhizosphere harbored many more bacterial operational taxonomic units and showed higher alpha diversity than the leaf and root endospheres. The relative abundance profiles of the bacterial community composition differed substantially between the compartments and populations, especially at the phylum and family levels. However, the top five phyla (Proteobacteria, Firmicutes, Bacteroidetes, Actinobacteria, and Acidobacteria) accounted for more than 90% of all the bacterial communities. Moreover, similar endophytic communities with a shared core set of bacteria were observed from different Senecio vulgaris populations. Heavy-metal-resistant, phosphate-solubilizing bacteria (Brevundimonas diminuta), nitrogen-fixing bacteria (Rhizobium leguminosarum), and cold-resistant bacteria (Exiguobacterium sibiricum) were present in the endosphere at relatively high abundance. This study, which reveals the structure of bacterial communities and their putative function in invasive Senecio vulgaris plants, is the first step in investigating the role of plant-bacteria interactions in the invasion of this species in China.

The Ecology and Evolution of Alien Plants

We review the state of the art of alien plant research with emphasis on conceptual advances and knowledge gains on general patterns and drivers, biotic interactions, and evolution. Major advances include the identification of different invasion stages and invasiveness dimensions (geographic range, habitat specificity, local abundance) and the identification of appropriate comparators while accounting for propagule pressure and year of introduction. Developments in phylogenetic and functional trait research bear great promise for better understanding of the underlying mechanisms. Global patterns are emerging with propagule pressure, disturbance, increased resource availability, and climate matching as major invasion drivers, but species characteristics also play a role. Biotic interactions with resident communities shape invasion outcomes, with major roles for species diversity, enemies, novel weapons, and mutualists. Mounting evidence has been found for rapid evolution of invasive aliens and evolutionary responses of natives, but a mechanistic understanding requires tighter integration of molecular and phenotypic approaches. We hope the open questions identified in this review will stimulate further research on the ecology and evolution of alien plants.

Contrasting effects of specialist and generalist herbivores on resistance evolution in invasive plants

Invasive alien plants are likely to be released from specialist herbivores and at the same time encounter biotic resistance from resident generalist herbivores in their new ranges. The Shifting Defense hypothesis predicts that this will result in evolution of decreased defense against specialist herbivores and increased defense against generalist herbivores. To test this, we performed a comprehensive meta-analysis of 61 common garden studies that provide data on resistance and/or tolerance for both introduced and native populations of 32 invasive plant species. We demonstrate that introduced populations, relative to native populations, decreased their resistance against specialists, and increased their resistance against generalists. These differences were significant when resistance was measured in terms of damage caused by the herbivore, but not in terms of performance of the herbivore. Furthermore, we found the first evidence that the magnitude of resistance differences between introduced and native populations depended significantly on herbivore origin (i.e., whether the test herbivore was collected from the native or non-native range of the invasive plant). Finally, tolerance to generalists was found to be higher in introduced populations, while neither tolerance to specialists nor that to simulated herbivory differed between introduced and native plant populations. We conclude that enemy release from specialist herbivores and biotic resistance from generalist herbivores have contrasting effects on resistance evolution in invasive plants. Our results thus provide strong support for the Shifting Defense hypothesis.

Host plant species determines symbiotic bacterial community mediating suppression of plant defenses

Herbivore associated bacteria are vital mediators of plant and insect interactions. Host plants play an important role in shaping the gut bacterial community of insects. Colorado potato beetles (CPB; Leptinotarsa decemlineata) use several Solanum plants as hosts in their natural environment. We previously showed that symbiotic gut bacteria from CPB larvae suppressed jasmonate (JA)-induced defenses in tomato. However, little is known about how changes in the bacterial community may be involved in the manipulation of induced defenses in wild and cultivated Solanum plants of CPB. Here, we examined suppression of JA-mediated defense in wild and cultivated hosts of CPB by chemical elicitors and their symbiotic bacteria. Furthermore, we investigated associations between the gut bacterial community and suppression of plant defenses using 16 S rRNA amplicon sequencing. Symbiotic bacteria decreased plant defenses in all Solanum hosts and there were different gut bacterial communities in CPB fed on different host plants. When larvae were reared on different hosts, defense suppression differed among host plants. These results demonstrate that host plants influence herbivore gut bacterial communities and consequently affect the herbivore’s ability to manipulate JA-mediated plant defenses. Thus, the presence of symbiotic bacteria that suppress plant defenses might help CPB adapt to host plants.

Impact of drought on plant populations of native and invasive origins

Invasive populations often shift phenotypically during introduction. Moreover, they are postulated to show an increased phenotypic plasticity compared with their native counterparts, which could be advantageous. However, less is known about trait selection across populations along the invasion gradient in response to environmental factors, such as increasing drought caused by climate change. In this study, we investigated the impacts of drought on growth, regrowth, and various leaf traits in plants of different origin. Therefore, seeds of 18 populations of the perennial Tanacetum vulgare were collected along the invasion gradient (North America, invasive; West Europe, archaeophyte; East Europe, native) and grown in competition with the grass Poa pratensis under control or dry conditions in a common garden. Above-ground biomass was cut once and the regrowth was measured as an indicator for tolerance over a second growth period. Initially, drought had little effects on growth of T. vulgare, but after cutting, plants grew more vigorously. Against expectations, phenotypic plasticity was not higher in invasive populations, but even reduced in one trait, which may be attributable to ecological constraints imposed by multiple stress conditions. Trait responses reflected the range expansion and invasion gradient and were influenced by the latitudinal origin of populations. Populations of invaded ranges may be subject to faster and more extensive genetic mixing or had less time to undergo and reflect selective processes.

The impact of plant chemical diversity on plant-herbivore interactions at the community level

Understanding the role of diversity in ecosystem processes and species interactions is a central goal of ecology. For plant-herbivore interactions, it has been hypothesized that when plant species diversity is reduced, loss of plant biomass to herbivores increases. Although long-standing, this hypothesis has received mixed support. Increasing plant chemical diversity with increasing plant taxonomic diversity is likely to be important for plant-herbivore interactions at the community level, but the role of chemical diversity is unexplored. Here we assess the effect of volatile chemical diversity on patterns of herbivore damage in naturally occurring patches of Piper (Piperaceae) shrubs in a Costa Rican lowland wet forest. Volatile chemical diversity negatively affected total, specialist, and generalist herbivore damage. Furthermore, there were differences between the effects of high-volatility and low-volatility chemical diversity on herbivore damage. High-volatility diversity reduced specialist herbivory, while low-volatility diversity reduced generalist herbivory. Our data suggest that, although increased plant diversity is expected to reduce average herbivore damage, this pattern is likely mediated by the diversity of defensive compounds and general classes of anti-herbivore traits, as well as the degree of specialization of the herbivores attacking those plants.

Defensive Traits in Young Pine Trees Cluster into Two Divergent Syndromes Related to Early Growth Rate

The combination of defensive traits leads to the evolution of 'plant defense syndromes' which should provide better protection against herbivores than individual traits on their own. Defense syndromes can be generally driven by plant phylogeny and/or biotic and abiotic factors. However, we lack a solid understanding of (i) the relative importance of shared evolution vs. convergence due to similar ecological conditions and (ii) the role of induced defense strategies in shaping defense syndromes. We investigate the relative roles of evolutionary and ecological factors shaping the deployment of pine defense syndromes including multiple constitutive and induced chemical defense traits. We performed a greenhouse experiment with seedlings of eighteen species of Pinaceae family, and measured plant growth rate, constitutive chemical defenses and their inducibility. Plant growth rate, but not phylogenetic relatedness, determined the deployment of two divergent syndromes. Slow-growing pine species living in harsh environments where tissue replacement is costly allocated more to constitutive defenses (energetically more costly to produce than induced). In contrast, fast-growing species living in resource-rich habitats had greater inducibility of their defenses, consistent with the theory of constitutive-induced defense trade-offs. This study contributes to a better understanding of evolutionary and ecological factors driving the deployment of defense syndromes.

Do Native Insects and Associated Fungi Limit Non-Native Woodwasp, Sirex noctilio, Survival in a Newly Invaded Environment?

Sirex noctilio F. (Hymenoptera: Siricidae) is an introduced pest of pines (Pinus spp.) in several countries in the Southern Hemisphere. Although S. noctilio is established in North America (first discovered in 2004), it has not been a destructive pest there so far, where forest communities more closely resemble those in its native Eurasian range—where it is not a pest. To investigate the influence of the existing community of associated insects (competitors + natural enemies) and fungi (vectored by insects) on S. noctilio survival in North America, we examined stage-specific mortality factors and their relative importance, generating life tables drawn from experimentally-manipulated and natural cohorts of Sirex spp. (mostly S. noctilio, but some native S. nigricornis F.). For both natural and experimentally-manipulated cohorts, factors which acted during the earliest Sirex life stages, most likely tree resistance and/or competition among fungal associates, were paramount in dictating woodwasp survival. Experimentally-manipulated life tables revealed that protection from the community of associates resulted in a significantly, and substantially larger (>15x) S. noctilio F1 generation than exposure to it. Seventy percent of generation mortality in the exposed cohort was due to tree resistance or unknown causes early in larval development, which could have included competition among other bark- or wood-inhabiting insects and/or their fungal associates. Only 46% of generation mortality in the protected cohort was due to tree resistance and/or unknown causes. Parasitoids, particularly endoparasitoids (Ibalia spp.), showed limited ability to control S. noctilio, and reduced the experimentally-established cohort by only 11%, and natural cohorts an average of 3.4%. The relative importance of tree resistance vs. competition with bark- and wood-borers in reducing S. noctilio survival remains unclear. Tree resistance and/or competition likely contribute more than natural enemies in maintaining the S. noctilio population in North America below damaging levels.

Specialist Insect Herbivore and Light Availability Do Not Interact in the Evolution of an Invasive Plant

Release from specialist insect herbivores may allow invasive plants to evolve traits associated with decreased resistance and increased competitive ability. Given that there may be genetic trade-off between resistance and tolerance, invasive plants could also become more tolerant to herbivores. Although it is widely acknowledged that light availability affects tolerance to herbivores, little information is available for whether the effect of light availability on tolerance differ between the introduced and native populations. We conducted a common garden experiment in the introduced range of Alternanthera philoxeroides using ten invasive US and ten native Argentinean populations at two levels of light availability and in the presence or absence of a specialist stem-boring insect Agasicles hygrophila. Plant biomass (total and storage root biomass), two allocation traits (root/shoot ratio and branch intensity, branches biomass/main stem biomass) and two functional traits (specific stem length and specific leaf area), which are potentially associated with herbivore resistance and light capture, were measured. Overall, we found that A. philoxeroides from introduced ranges had comparable biomass and tolerance to specialist herbivores, lower branch intensity, lower specific stem length and specific leaf area. Moreover, introduced populations displayed higher shade tolerance of storage root biomass and lower plastic response to shading in specific stem length. Finally, light availability had no significant effect on evolution of tolerance to specialist herbivores of A. philoxeroides. Our results suggest that post-introduction evolution might have occurred in A. philoxeroides. While light availability did not influence the evolution of tolerance to specialist herbivores, increased shade tolerance and release from specialist insects might have contributed to the successful invasion of A. philoxeroides.

Evolution of plant growth and defense in a continental introduction

Substantial research has addressed adaptation of nonnative biota to novel environments, yet surprisingly little work has integrated population genetic structure and the mechanisms underlying phenotypic differentiation in ecologically important traits. We report on studies of the common milkweed Asclepias syriaca, which was introduced from North America to Europe over the past 400 years and which lacks most of its specialized herbivores in the introduced range. Using 10 populations from each continent grown in a common environment, we identified several growth and defense traits that have diverged, despite low neutral genetic differentiation between continents. We next developed a Bayesian modeling approach to account for relationships between molecular and phenotypic differences, confirming that continental trait differentiation was greater than expected from neutral genetic differentiation. We found evidence that growth-related traits adaptively diverged within and between continents. Inducible defenses triggered by monarch butterfly herbivory were substantially reduced in European populations, and this reduction in inducibility was concordant with altered phytohormonal dynamics, reduced plant growth, and a trade-off with constitutive investment. Freedom from the community of native and specialized herbivores may have favored constitutive over induced defense. Our replicated analysis of plant growth and defense, including phenotypically plastic traits, suggests adaptive evolution following a continental introduction.

Genotype × herbivore effect on leaf litter decomposition in Betula Pendula saplings: ecological and evolutionary consequences and the role of secondary metabolites

Plant genetic variation and herbivores can both influence ecosystem functioning by affecting the quantity and quality of leaf litter. Few studies have, however, investigated the effects of herbivore load on litter decomposition at plant genotype level. We reduced insect herbivory using an insecticide on one half of field-grown Betula Pendula saplings of 17 genotypes, representing random intrapopulation genetic variation, and allowed insects to naturally colonize the other half. We hypothesized that due to induced herbivore defence, saplings under natural herbivory produce litter of higher concentrations of secondary metabolites (terpenes and soluble phenolics) and have slower litter decomposition rate than saplings under reduced herbivory. We found that leaf damage was 89 and 53% lower in the insecticide treated saplings in the summer and autumn surveys, respectively, which led to 73% higher litter production. Litter decomposition rate was also affected by herbivore load, but the effect varied from positive to negative among genotypes and added up to an insignificant net effect at the population level. In contrast to our hypothesis, concentrations of terpenes and soluble phenolics were higher under reduced than natural herbivory. Those genotypes, whose leaves were most injured by herbivores, produced litter of lowest mass loss, but unlike we expected, the concentrations of terpenes and soluble phenolics were not linked to either leaf damage or litter decomposition. Our results show that (1) the genetic and herbivore effects on B. pendula litter decomposition are not mediated through variation in terpene or soluble phenolic concentrations and suggest that (2) the presumably higher insect herbivore pressure in the future warmer climate will not, at the ecological time scale, affect the mean decomposition rate in genetically diverse B. pendula populations. However, (3) due to the significant genetic variation in the response of decomposition to herbivory, evolutionary changes in mean decomposition rate are possible.

Diversity of pyrrolizidine alkaloids in native and invasive Senecio pterophorus (Asteraceae): implications for toxicity

Changes in plant chemical defenses after invasion could have consequences on the invaded ecosystems by modifying the interactions between plants and herbivores and facilitating invasion success. However, no comprehensive biogeographical studies have yet determined the phenotypic levels of plant chemical defenses, as consumed by local herbivores, covering large distributional areas of a species. Senecio pterophorus is a perennial shrub native to Eastern South Africa, expanded into Western South Africa and introduced into Australia and Europe. As other Asteraceae, S. pterophorus contains pyrrolizidine alkaloids (PAs) toxic to vertebrate and invertebrate herbivores. Here we analyzed S. pterophorus PAs by LC-MS/MS on foliage sampled across its entire distributional range, including the native and all non-native areas. PA concentrations and diversity was very high: we found 57 compounds belonging to 6 distinct necine base-types, including the highly toxic 1,2-unsaturated PAs (retronecine and otonecines) and the less toxic 1,2-saturated PAs (platynecine and rosmarinecines). Plants from different origins diverged in their PA absolute and relative concentrations. Rosmarinine was the most abundant compound in Australia and South Africa, but it was nearly absent in Europe. We characterized three plant chemotypes: retrorsine-senkirkine chemotype in Eastern South Africa, rosmarinine chemotype in Australia and Western South Africa, and acetylseneciphylline chemotype in Europe. PA absolute concentrations were highest in Australia. The increased absolute and relative concentrations of retronecine PAs from Australia and Europe, respectively, indicate that S. pterophorus is potentially more toxic in the invasive range than in the native range.

Activated carbon decreases invasive plant growth by mediating plant-microbe interactions

There is growing appreciation for the idea that plant-soil interactions (e.g. allelopathy and plant-microbe feedbacks) may explain the success of some non-native plants. Where this is the case, native plant restoration may require management tools that change plant-soil interactions. Activated carbon (AC) is one such potential tool. Previous research has shown the potential for high concentrations of AC to restore native plant growth to areas dominated by non-natives on a small scale (1 m × 1 m plots). Here we (i) test the efficacy of different AC concentrations at a larger scale (15 m × 15 m plots), (ii) measure microbial responses to AC treatment and (iii) use a greenhouse experiment to identify the primary mechanism, allelopathy versus microbial changes, through which AC impacts native and non-native plant growth. Three years after large-scale applications, AC treatments decreased non-native plant cover and increased the ratio of native to non-native species cover, particularly at concentrations >400 g m(-2). Activated carbon similarly decreased non-native plant growth in the greenhouse. This effect, however, was only observed in live soils, suggesting that AC effects were microbially mediated and not caused by direct allelopathy. Bacterial community analysis of field soils indicated that AC increased the relative abundance of an unidentified bacterium and an Actinomycetales and decreased the relative abundance of a Flavobacterium, suggesting that these organisms may play a role in AC effects on plant growth. Results support the idea that manipulations of plant-microbe interactions may provide novel and effective ways of directing plant growth and community development (e.g. native plant restoration).

Densities of Agrilus auroguttatus and Other Borers in California and Arizona Oaks

We investigated within-tree population density of a new invasive species in southern California, the goldspotted oak borer, Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), with respect to host species and the community of other borers present. We measured emergence hole densities of A. auroguttatus and other borers on the lower stem (bole) of naïve oaks at 18 sites in southern California and on co-evolved oaks at seven sites in southeastern Arizona. We sampled recently dead oaks in an effort to quantify the community of primary and secondary borers associated with mortality—species that were likely to interact with A. auroguttatus. Red oaks (Section Lobatae) produced greater densities of A. auroguttatus than white oaks (Section Quercus). On red oaks, A. auroguttatus significantly outnumbered native borers in California (mean ± SE of 9.6 ± 0.7 versus 4.5 ± 0.6 emergence holes per 0.09 m² of bark surface), yet this was not the case in Arizona (0.9 ± 0.2 versus 1.1 ± 0.2 emergence holes per 0.09 m²). In California, a species that is taxonomically intermediate between red and white oaks, Quercus chrysolepis (Section Protobalanus), exhibited similar A. auroguttatus emergence densities compared with a co-occurring red oak, Q. kelloggii. As an invasive species in California, A. auroguttatus may affect the community of native borers (mainly Buprestidae and Cerambycidae) that feed on the lower boles of oaks, although it remains unclear whether its impact will be positive or negative.

Loss of specificity: native but not invasive populations of Triadica sebifera vary in tolerance to different herbivores

During introduction, invasive plants can be released from specialist herbivores, but may retain generalist herbivores and encounter novel enemies. For fast-growing invasive plants, tolerance of herbivory via compensatory regrowth may be an important defense against generalist herbivory, but it is unclear whether tolerance responses are specifically induced by different herbivores and whether specificity differs among native and invasive plant populations. We conducted a greenhouse experiment to examine the variation among native and invasive populations of Chinese tallow tree, Triadica sebifera, in their specificity of tolerance responses to herbivores by exposing plants to herbivory from either one of two generalist caterpillars occurring in the introduced range of Triadica. Simultaneously, we measured the specificity of another defensive trait, extrafloral nectar (EFN) production, to detect potential tradeoffs between resistance and tolerance of herbivores. Invasive populations had higher aboveground biomass tolerance than native populations, and responded non-specifically to either herbivore, while native populations had significantly different and specific aboveground biomass responses to the two herbivores. Both caterpillar species similarly induced EFN in native and invasive populations. Plant tolerance and EFN were positively correlated or had no relationship and biomass in control and herbivore-damaged plants was positively correlated, suggesting little costs of tolerance. Relationships among these vegetative traits depended on herbivore type, suggesting that some defense traits may have positive associations with growth-related processes that are differently induced by herbivores. Importantly, loss of specificity in invasive populations indicates subtle evolutionary changes in defenses in invasive plants that may relate to and enhance their invasive success.

Chinese tallow trees (Triadica sebifera) from the invasive range outperform those from the native range with an active soil community or phosphorus fertilization

Two mechanisms that have been proposed to explain success of invasive plants are unusual biotic interactions, such as enemy release or enhanced mutualisms, and increased resource availability. However, while these mechanisms are usually considered separately, both may be involved in successful invasions. Biotic interactions may be positive or negative and may interact with nutritional resources in determining invasion success. In addition, the effects of different nutrients on invasions may vary. Finally, genetic variation in traits between populations located in introduced versus native ranges may be important for biotic interactions and/or resource use. Here, we investigated the roles of soil biota, resource availability, and plant genetic variation using seedlings of Triadica sebifera in an experiment in the native range (China). We manipulated nitrogen (control or 4 g/m(2)), phosphorus (control or 0.5 g/m(2)), soil biota (untreated or sterilized field soil), and plant origin (4 populations from the invasive range, 4 populations from the native range) in a full factorial experiment. Phosphorus addition increased root, stem, and leaf masses. Leaf mass and height growth depended on population origin and soil sterilization. Invasive populations had higher leaf mass and growth rates than native populations did in fresh soil but they had lower, comparable leaf mass and growth rates in sterilized soil. Invasive populations had higher growth rates with phosphorus addition but native ones did not. Soil sterilization decreased specific leaf area in both native and exotic populations. Negative effects of soil sterilization suggest that soil pathogens may not be as important as soil mutualists for T. sebifera performance. Moreover, interactive effects of sterilization and origin suggest that invasive T. sebifera may have evolved more beneficial relationships with the soil biota. Overall, seedlings from the invasive range outperformed those from the native range, however, an absence of soil biota or low phosphorus removed this advantage.

Specificity of extrafloral nectar induction by herbivores differs among native and invasive populations of tallow tree

BACKGROUND AND AIMS

Invasive plants can be released from specialist herbivores and encounter novel generalists in their introduced ranges, leading to variation in defence among native and invasive populations. However, few studies have examined how constitutive and induced indirect defences change during plant invasion, especially during the juvenile stage.

METHODS

Constitutive extrafloral nectar (EFN) production of native and invasive populations of juvenile tallow tree (Triadica sebifera) were compared, and leaf clipping, and damage by a native specialist (Noctuid) and two native generalist caterpillars (Noctuid and Limacodid) were used to examine inducible EFN production.

KEY RESULTS

Plants from introduced populations had more leaves producing constitutive EFN than did native populations, but the content of soluble solids of EFN did not differ. Herbivores induced EFN production more than simulated herbivory. The specialist (Noctuid) induced more EFN than either generalist for native populations. The content of soluble solids in EFN was higher (2·1 times), with the specialist vs. the generalists causing the stronger response for native populations, but the specialist response was always comparable with the generalist responses for invasive populations.

CONCLUSIONS

These results suggest that constitutive and induced indirect defences are retained in juvenile plants of invasive populations even during plant establishment, perhaps due to generalist herbivory in the introduced range. However, responses specific to a specialist herbivore may be reduced in the introduced range where specialists are absent. This decreased defence may benefit specialist insects that are introduced for classical biological control of invasive plants.

How slug herbivory of juvenile hybrid willows alters chemistry, growth and subsequent susceptibility to diverse plant enemies

BACKGROUND AND AIMS

Selective feeding by herbivores, especially at the seedling or juvenile phase, has the potential to change plant traits and ultimately the susceptibility of surviving plants to other enemies. Moreover, since hybridization is important to speciation and can lead to introgression of traits between plant species, differential feeding (herbivore-induced mortality) can influence the expression of resistance traits of hybrids and ultimately determine the consequences of hybridization. While it would be expected that herbivore-induced mortality would lead to greater resistance, there may be trade-offs whereby resistance to one herbivore increases susceptibility to others. The hypothesis was tested that the exotic slug, Arion subfuscus, causes non-random survival of hybrid willows and alters plant: (1) susceptibility to slugs; (2) secondary and nutritional chemistry, and growth; and (3) susceptibility to other phytophages.

METHODS

Two populations of plants, control and selected, were created by placing trays of juvenile willows in the field and allowing slugs access to only some. When ≤10 individuals/tray remained (approx. 85 % mortality), 'selected' and undamaged 'control' trays were returned to a common area. Traits of these populations were then examined in year 1 and in subsequent years.

KEY RESULTS

The selected population was less palatable to slugs. Surprisingly, foliar concentrations of putative defence traits (phenolic glycosides and tannins) did not differ between treatments, but the selected population had higher foliar nitrogen and protein, lower carbon to nitrogen ratio and greater above-ground biomass, indicating that vigorously growing plants were inherently more resistant to slugs. Interestingly, selected plants were more susceptible to three phytophages: an indigenous pathogen (Melampsora epitea), a native herbivorous beetle (Chrysomela knabi) and an exotic willow leaf beetle (Plagiodera versicolora).

CONCLUSIONS

This exotic slug changed the population structure of F2 hybrid willows in unanticipated ways. Defence expression remained unchanged, while nutritional and growth traits changed. These changes caused plants to be more susceptible to other plant enemies. Other exotic herbivore species are anticipated to have similar direct and indirect effects on native plant populations.

Additive genetic variation in resistance traits of an exotic pine species: little evidence for constraints on evolution of resistance against native herbivores

The apparent failure of invasions by alien pines in Europe has been explained by the co-occurrence of native pine congeners supporting herbivores that might easily recognize the new plants as hosts. Previous studies have reported that exotic pines show reduced tolerance and capacity to induce resistance to those native herbivores. We hypothesize that limited genetic variation in resistance to native herbivores and the existence of evolutionary trade-offs between growth and resistance could represent additional potential constraints on the evolution of invasiveness of exotic pines outside their natural range. In this paper, we examined genetic variation for constitutive and induced chemical defences (measured as non-volatile resin in the stem and total phenolics in the needles) and resistance to two major native generalist herbivores of pines in cafeteria bioassays (the phloem-feeder Hylobius abietis and the defoliator Thaumetopoea pityocampa) using half-sib families drawn from a sample of the population of Pinus radiata introduced to Spain in the mid-19th century. We found (i) significant genetic variation, with moderate-to-high narrow-sense heritabilities for both the production of constitutive non-volatile resin and induced total phenolics, and for constitutive resistance against T. pityocampa in bioassays, (ii) no evolutionary trade-offs between plant resistance and growth traits or between the production of different quantitative chemical defences and (iii) a positive genetic correlation between constitutive resistance to the two studied herbivores. Overall, results of our study indicate that the exotic pine P. radiata has limited genetic constraints on the evolution of resistance against herbivores in its introduced range, suggesting that, at least in terms of interactions with these enemies, this pine species has potential to become invasive in the future.